The storage, handling, and transfer of high level waste, (hereinafter, “HLW”) such as spent nuclear fuel (hereinafter, “SNF”), requires special care and procedural safeguards. For example, in the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted down to a predetermined level. Upon removal, this spent nuclear fuel is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. In order to protect the environment from radiation exposure, spent nuclear fuel is first placed in a canister. The loaded canister is then transported and stored in large cylindrical containers called casks. A transfer cask is used to transport spent nuclear fuel from location to location while a storage cask is used to store spent nuclear fuel for a determined period of time.
In a typical nuclear power plant, an open empty canister is first placed in an open transfer cask. The transfer cask and empty canister are then submerged in a pool of water. Spent nuclear fuel is loaded into the canister while the canister and transfer cask remain submerged in the pool of water. Once fully loaded with spent nuclear fuel, a lid is typically placed atop the canister while in the pool. The transfer cask and canister are then removed from the pool of water, the lid of the canister is welded thereon and a lid is installed on the transfer cask. The canister is then properly dewatered and filled with inert gas. The transfer cask (which is holding the loaded canister) is then transported to a location where a storage cask is located. The loaded canister is then transferred from the transfer cask to the storage cask for long term storage. During transfer from the transfer cask to the storage cask, it is imperative that the loaded canister is not exposed to the environment.
One type of storage cask is a ventilated vertical overpack (“VVO”). A VVO is a massive structure made principally from steel and concrete and is used to store a canister loaded with spent nuclear fuel (or other HLW). VVOs stand above ground and are typically cylindrical in shape and extremely heavy, weighing over 150 tons and often having a height greater than 16 feet. VVOs typically have a flat bottom, a cylindrical body having a cavity to receive a canister of spent nuclear fuel, and a removable top lid.
In using a VVO to store spent nuclear fuel, a canister loaded with spent nuclear fuel is placed in the cavity of the cylindrical body of the VVO. Because the spent nuclear fuel is still producing a considerable amount of heat when it is placed in the VVO for storage, it is necessary that this heat energy have a means to escape from the VVO cavity. This heat energy is removed from the outside surface of the canister by ventilating the VVO cavity. In ventilating the VVO cavity, cool air enters the VVO chamber through bottom ventilation ducts, flows upward past the loaded canister, and exits the VVO at an elevated temperature through top ventilation ducts. The bottom and top ventilation ducts of existing VVOs are located near the bottom and top of the VVO's cylindrical body respectively.
While it is necessary that the VVO cavity be vented so that heat can escape from the canister, it is also imperative that the VVO provide adequate radiation shielding and that the spent nuclear fuel not be directly exposed to the external environment. The inlet duct located near the bottom of the overpack is a particularly vulnerable source of radiation exposure to security and surveillance personnel who, in order to monitor the loaded overpacks, must place themselves in close vicinity of the ducts for short durations. Thus, a need exists for a VVO system for the storage of high level radioactive waste that has an inlet duct that reduces the likelihood of radiation exposure while providing extreme radiation blockage of both gamma and neutron radiation emanating from the high level radioactive waste.
The effect of wind on the thermal performance of a ventilated system can also be a serious drawback that, to some extent, afflicts all systems in use in the industry at the present time. Storage VVO's with only two inlet or outlet ducts are especially vulnerable. While axisymmetric air inlet and outlet ducts behave extremely well in quiescent air, when the wind is blowing, the flow of air entering and leaving the system is skewed, frequently leading to a reduced heat rejection capacity.
The thick top lid is one of the most expensive components of a radioactive waste canister. Such canisters may be used to store and transport non-fuel radioactive waste from nuclear generation plants such as activated reactor internals, control components, sundry non-fissile materials, and waste from operations such as resins, and in some applications vitrified nuclear waste fuel (“glass logs”) encased in an outer metal cylinder. On existing canisters, the thick top lid is needed to shield personnel from radiation who are working on the lid (e.g. welding, bolting, fluid operations, etc.). The lid must also be thicker because the lid further performs the main canister lifting connection, and therefore must have the thickness needed for structural reasons to support the weight of the entire canister when hoisted via a crane or similar equipment used to move the canister. For these reasons, the thick top lid of a waste canister adds considerably to the overall weight and expense of the canister. An improved radioactive waste canister is desired.
A need also exists periodic leak testing is often required for monitoring the integrity of the inner and outer confinement boundaries on canisters holding radioactive materials. Some present leak testing processes involve removing the cask lid, which is undesirable, as doing so has the potential to increase radiation exposure to workers. Other leak testing processes and systems involve installing a continuous leak testing monitoring system that uses a compressed helium tank and pressure transducers. Such a system, however, requires periodic replacement of the transducers and replenishment of the helium gas stored in the tank. In view of the shortcomings of present leak detection processes and systems, improvements are desirable which reduce the on-site maintenance requirements, improve leak detection capabilities, and reduce potential radiation exposure to workers.
A need also exists for the ability to better examine welds formed on containers that are used to store spend nuclear fuel. Finally, a need exists to better enable spent nuclear fuel to be transferred from place to place as necessary.